IP Subnet Calculator

They answer different questions: this tool tells you which equal subnets a block can be cut into — enter the parent network and a split count, and it lists each subnet's network address, mask, and usable range — while cidr-calculator focuses on the subnet details of a single CIDR (network address, broadcast address, mask, and so on). Planning usually fixes the total block size first, then comes here to split.

Yes. You can derive masks and address ranges from "I need N subnets" or "N hosts per subnet". Note that subnet sizes must align to powers of two — needing three pieces usually means cutting four and keeping one spare. Illegal addresses, out-of-range prefixes, and non-alignable split counts all raise explicit errors instead of silently producing a wrong table.

This page focuses on equal splits. VLSM discipline is "largest first, aligned boundaries": allocate in descending host-count order, with each new segment starting on a multiple of its own mask. Skipping the descending order leaves fragmentation you can only fix by renumbering — verify each segment here or on the routing calculator page.

Aggregation requires contiguous addresses, equal block sizes, and a start aligned to the aggregated mask — four contiguous, aligned /26s merge into one /24, for example. The most common failure is unequal block sizes rather than gaps, so equalize the split before aggregating.

Vendor-reserved addresses make the theoretically usable range larger than what the console shows, so leave headroom. Reference a segment in security groups as network address plus prefix, never a single host address standing in for the network. And export the split as a table for change review — it prevents far more collisions than verbally asking for "a few more /24s".